Metallic compounds, which are exposed to high temperature must be protected against heat and corrosion. This is especially true for parts of gas turbines like combustion chambers, turbine blades or vanes. These parts are commonly coated with an intermediate MCrAlY layer (M=Fe, Co, Ni) and a thermal barrier coating (TBC) which is applied on top of the intermediate layer. Between the two layers an aluminium oxide layer is formed due to oxidation.
The bonding of the three different layers is crucial for high durability of the protection layer as a whole. Problems may arise, if there are big differences in the thermal expansion factors of the different layers. In this case failure of the thermal barrier coating might occur, which can lead to the destruction of the whole compound.
From U.S. Pat. No. 6,287,644 a continuously graded MCrAlY bond coat is known which has a continuously increasing amount of Cr, Si or Zr with increasing distance from the underlaying substrate in order to reduce the thermal mismatch between the bond coat and the thermal barrier coating by adjusting the thermal expansion factors.
The U.S. Pat. No. 5,792,521 shows a multi layer thermal barrier coating.
U.S. Pat. No. 5,514,482 discloses a thermal barrier coating system for super alloy components, in which the MCrAlY layer is substituted by an aluminium coating layer such as NiAl. In order to obtain the desired properties the NiAl layer has to be quite thick because of its brittleness.
From EP 1 082 216 B1 a MCrAlY layer is known, which has the γ-phase at its outer layer. This γ-phase can only be obtained by remelting or deposition from a liquid phase in an expensive way.
EP 1 380 672 A1 discloses a highly oxidation resistant component with a protective layer, which consists of an intermediate MCrAlY layer zone and an outer layer zone, which has the structure of the phase β-NiAl.
The layer systems mentioned above are either expensive or lack a strong bonding between the different layer zones.